Macromolecular Rapid Communications
Communication
Comonomer-Induced Stereo-Selectivity Enhancement in a C2-Symmetric MetalloceneCatalyzed Propylene Polymerization Lin Ma, Jin-Yong Dong*
Propylene polymerization is carried out with a C2-symmetric metallocene catalyst of racEt(Ind)2ZrCl2/MAO at 40 °C in the presence of a cyclo-triene of trans,trans,cis-1,5,9-cyclododecatriene ((E,E,Z)-CDT). Comonomer incorporations are rather low (7% in [mmmm]). (E,E,Z)-CDT is speculated to coordinate to the metal center forming comonomer-complexed active sites in charge of the entire polymerization reaction with decreased activity however increased propylene enantiomorphic selectivity.
1. Introduction Stereochemistry control is one of the most basic issues in polymer chemistry.[1] In coordination polymerization of α-olefins, it is particularly important as, on one hand, coordination polymerization allows the synthesis of stereoregular poly(α-olefin)s (e.g., isotactic polypropylene) by regulating the enantiofaces of the monomers during their insertions, on the other hand, poly(α-olefin)s, or polyolefins, with high stereo-regularity are prone to crystallize
L. Ma, Prof. J.-Y. Dong CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China E-mail: [email protected] L. Ma University of Chinese Academy of Sciences, Beijing 100049, China Macromol. Rapid Commun. 2015, 36, 733−738 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
so as to make such polyolefins as polypropylene of weak interchain interaction really useful polymeric materials with remarkable mechanical- and thermal-resistant properties.[2] In general, stereo-selectivity of α-olefin coordination polymerization originates from the transition metal catalyst that enables the polymerization. For isospecific propylene polymerization enabling isotactic polypropylene, it is known that the development of C2-symmetric ansa-metallocene catalysts with an enantiomorphic-site control mechanism has been so successful that some of the state-of-the-art ones are now able to rival their commercial heterogeneous counterparts in producing highly isotactic polypropylenes ([mmmm] > 99%) only with far higher activities.[3–8] However, it has been reported, even with the sitecontrolled C2-symmetric ansa-metallocene catalysts, the stereo-selectivity of propylene insertions will be affected by the last-inserted monomer unit when a comonomer is introduced into the polymerization.[9–11] Sacchi et al.,
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for one instance, found that the presence of 1-pentene during propylene polymerization by rac-Me2Si(2-MeBenz[e]Ind)2ZrCl2/MAO would decrease its stereo-selectivity, forming copolymers of gradually reduced isotacticities.[9] It was proposed that the insertion of a higher α-olefin comonomer such as 1-pentene would trigger a shift of the catalyst active site from highly stereo-selective to less or even non-stereo-selective.[10] For another instance, Tritto and co-workers observed that copolymerization with norbornene also greatly reduced the tacticity of the PP blocks when propylene copolymerization was carried out with rac-Me2Si(2-Me-Ind)2ZrCl2/MAO.[11] Meanwhile, it was found that, with rac-Et(Ind)2ZrCl2 as catalyst, both copolymerizations have little effect on its stereo-selectivity, the formed polypropylene possessing comparable isotacticities as well as similar stereo-sequences when related to polypropylene homopolymer. Concerning the effect of copolymerization on the stereo-selectivity of C2-symmetric metallocene catalysts, it is however noticed that there has been so far no report of catalyst stereo-selectivity increasing by copolymerization. In this report, we design a special copolymerization of metallocene-catalyzed propylene polymerization using a bulky cyclo-triene of trans,trans,cis-1,5,9-cyclododecatriene ((E,E,Z)-CDT) as comonomer. With racEt(Ind)2ZrCl2/MAO as a catalyst complex, we will show the first example of increasing polypropylene tacticity by a comonomer of (E,E,Z)-CDT.
2. Results and Discussion The cyclo-triene comonomer, (E,E,Z)-CDT, is a multifunctional cyclic olefin taking a peculiar configuration as shown in Figure 1.[12] Its 1H NMR spectrum is included in Figure 2 (Figure 2a), where the characteristic chemical shifts can be grouped to high-field and low-field ones, corresponding, respectively, to the cyclic methylene and vinylic methine protons. The one cis double bond exhibits as a single proton resonance at 5.28 ppm, whereas the trans double bonds are revealed by two distinct peaks at 5.19 and 5.03 ppm. The copolymerization of propylene and (E,E,Z)-CDT was carried out in toluene with the catalyst system composed of rac-Et(Ind)2ZrCl2 and MAO. A relatively low temperature of 40 °C was chosen for the copolymerization for restraining the chain transfer reactions.[13] The concentrations of (E,E,Z)-CDT were gradually increased in the series of polymerization runs. The resultant polymer samples were worked up by a thorough washing with ethanol after they had been retrieved from the polymerization reactor, to assure complete removal of unreacted (E,E,Z)-CDT. Table 1 summarizes the polymerization conditions and results. The obtained copolymers were first examined by 1H NMR, the spectra of which are shown in Figure 2, where
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Figure 1. Structure of trans,trans,cis-1,5,9-cyclododecatriene, (E,E,Z)-CDT.
they are being compared with the monomeric (E,E,Z)-CDT as well as with the PP homopolymer (Figure 2b) to detect any possible (E,E,Z)-CDT incorporations. As demonstrated in Figure 2, although weak, the characteristic resonances of the vinylic protons of (E,E,Z)-CDT around 5.0–5.3 ppm can still be distinguished from interference by those of the vinylidene protons of the chain terminals resulting from β-H elimination. In fact, this set of peaks displays a similar contour as those in the monomeric form. The cis double bond protons are shown with a resonance signal around 5.3 ppm, which indicates enchainment of the trans double bond. Signals around 5.03 and 5.19 ppm are from the trans double bond protons. The emergence of all these resonance signals suggests that both the cis and trans double bonds of (E,E,Z)-CDT are able to be engaged in copolymerization with propylene. And a rough integration of the individual peaks reveals that the cis and trans double bonds maintain a 1/2 ratio as that in its monomeric form, from where it can be assumed that the two types of double bonds may have similar reactivity. Considering the large molecular bulk of (E,E,Z)-CDT which is expected of considerable steric hindrance, it is thus temporally assumed that for each incorporated (E,E,Z)-CDT only one double bond would be enchained. Thus also by peak integration, the relative contents of (E,E,Z)-CDT in polymers can be estimated, the results included in Table 1. It has to be admitted that, with the current catalyst system and under the current polymerization conditions, the incorporations of (E,E,Z)-CDT are very limited, the maximum being only 0.10 mol%. These tiny amounts of incorporations are easy to be mistaken as that unreacted monomers failed to remove during polymer work-up process. To clarify this ambiguity, all samples were subjected to rigorous boiling ethanol extraction for extensive hours before tested again by 1H NMR. The results turned out to be almost identical as the previous ones, which thus verifies the authenticity of (E,E,Z)-CDT incorporations in polymers, though again extremely small. Despite these low incorporations of (E,E,Z)-CDT, as apparent from Table 1, substantial losses of
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Macromolecular Rapid Communications
Comonomer-Induced Stereo-Selectivity Enhancement in a C2-Symmetric Metallocene-Catalyzed Propylene Polymerization
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Figure 2. 1H NMR spectra of a) monomeric (E,E,Z)-CDT, b) pristine PP (run 1 in Table 1), and c–e) its (E,E,Z)-CDT copolymers with different (E,E,Z)-CDT incorporations. c) Run 2 in Table 1; d) run 3 in Table 1; e) run 4 in Table 1.
catalyst activity were encountered during the copolymerization, and the higher the (E,E,Z)-CDT feed in the copolymerization, the lower the catalyst activity. Molecular weights (Mw) as well as molecular weight distributions (PDI) of the copolymers were determined by
GPC. The results are also included in Table 1. It has been known that, in case of propylene/α-olefin copolymerization with the same catalyst system, the copolymer molecular weights would decrease with increasing of copolymer’s α-olefin content due to more frequent chain transfer
Table 1. Conditionsa) and results of copolymerization of propylene and (E,E,Z)-CDT with rac-Et(Ind)2ZrCl2/MAO. Run
CDT [mol L−1]
Yield [g]
Cat. activityb)
CDT in polymer [mol%]
Mwc) [g mol−1]
PDIc)
Tmd) [°C]
ΔHfd) [J g−1]
Tcd) [°C]
Stereo-pentade) [%] [mmmm]
[mmmr]
[mmrr]
[mrrm]
1
0
4.9
4.9
0
27 700
2.8
131.7
54.7
101.0
82.1
8.0
6.4
3.2
2
0.5
2.5
2.5
≈0.04
27 500
3.1
134.6
66.4
104.6
88.7
5.9
4.3
1.1
3
1.0
1.3
1.3
≈0.06
21 500
2.8
134.8
68.7
105.8
86.0
7.0
5.0
1.5
4
1.5
0.6
0.6
≈0.10
26 400
2.9
135.1
76.3
104.6
89.3
6.2
3.4
0.9
a) Zr = 2 μmol, molar ratio of (MAO) Al/Zr = 2000, CDT + toluene = 50 mL, propylene = 0.1 MPa, polymerization temperature = 40 °C, polymerization duration = 0.5 h; b)×106 g mol−1 Zr h; c)Determined by GPC; d)Determined by DSC; e)Determined by 13C NMR.
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reaction at α-olefin-inserted propagating chain end.[13] However, comparing the PP/(E,E,Z)-CDT copolymers with the PP homopolymer in Table 1, no notable decrease of Mw can be found. In fact, this is a highly valued information for it rules out such speculations as that the comonomer might be placed at the polymer chain end. Besides molecular weight, the copolymers also maintain relatively narrow molecular weight distributions. The GPC curves (shown in the Supporting Information) are smooth without any shoulder peaks. The Mark–Houwink plots (also shown in the Supporting Information) indicate that all copolymer samples exhibit straight line log[η] versus logMw relationship parallel with that of the homopolymer sample, suggesting that there is no branching structure formed in the copolymers, which thus confirms the previous hypothesis that each enchained (E,E,Z)-CDT has only one double bond, be it the cis or one of the two trans ones, engaged in the copolymerization. The PP block stereo-sequences in PP/(E,E,Z)-CDT copolymers were examined and quantified by 13C NMR, in comparison with those of the reference PP homopolymer (Figure 3). The copolymers display, similar to the PP homopolymer, three major stereo-irregular sequences including mmmr, mmrr, and mrrm, along with the stereoregular mmmm pentad. Interestingly, quantification of these stereo-sequences results in an unexpected finding that copolymerization with (E,E,Z)-CDT even increases isotacticity in PP, which in turn suggests stereo-selectivity enhancement for this otherwise well-known metallocene catalyst. Comparing the mmmm pentad percentage fraction in the whole stereo-sequences between the reference PP homopolymer and the PP/(E,E,Z)-CDT copolymers and
among the copolymers themselves, an up to 7.2% increment from 82.1% in pristine PP homopolymer to 89.3% in copolymer containing only 0.1 mol% of (E,E,Z)-CDT is evidenced enough to showcase the remarkable effect of (E,E,Z)-CDT the cyclo-triene as comonomer on the stereoselectivity of the metallocene-catalyzed polymerization. Increases in PP isotacticity lead to improved melt crystallization properties. Differential scanning calorimetry (DSC) thermogram comparison (shown in the Supporting Information) is made between the pristine PP homopolymer and PP/(E,E,Z)-CDT copolymers at both heating and cooling cycles. Some important data from this comparison are collected and included in Table 1. Again, with so small comonomer incorporations, melting temperatures (Tm) of PP/(E,E,Z)-CDT copolymers increase from 131.0 °C of PP homopolymer to 135.8 °C of copolymer with 0.1 mol% of (E,E,Z)-CDT enchainment. In accordance, melting fusion enthalpies (ΔHf ) increase from 54.0 to 76.8 J g−1, and crystallization temperatures (Tc) from 101.1 to ≈105.1 °C. These notable increments in crystallization properties are expected to induce significant improvements in mechanical and processing properties of ultimate PP applications. To summarize the copolymerization of propylene with (E,E,Z)-CDT under the as-adopted conditions, we have found that, despite the copolymerization has not been so successful since comonomer incorporations are all too limited, it is still of great scientific interest for it provides the very first example of comonomer inducing phenomenal stereo-selectivity enhancement in coordination α-olefin polymerization using “site-controlled” C2-symmetric metallocene catalyst. To account for this exceptional experimental finding, we quote two related previous works both by Sacchi et al. that are highly insightful in terms of explaining exotic influences in metallocene-catalyzed olefin polymerization.[9–11,14,15] In one of the two works they proposed,[9–11] as mentioned earlier, that in propylene/α-olefin (e.g., 1-pentene) copolymerization resolved by racMe 2 Si(2-MeBenz-[ e ]Ind) 2 ZrCl 2 /MAO, insertion of the α-olefin comonomer at the propagating polymer chain end would have a temporal influence on the active catalyst structure that affects the stereo-selectivity of the immediately followed several propylene monomer insertions (a downwards effect). In the other,[14,15] with abundant evidences, they have asserted polymerization medium, that is, solvent which is coordinative with donor character such as Figure 3. 13C NMR spectra at the methyl region of a) pristine PP (run 1 in Table 1) and b–d) its (E,E,Z)-CDT copolymers with different (E,E,Z)-CDT incorporations. b) Run 2 in toluene or mesitylene will pose competition with α-olefin monomers during Table 1; c) run 3 in Table 1; d) run 4 in Table 1.
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Comonomer-Induced Stereo-Selectivity Enhancement in a C2-Symmetric Metallocene-Catalyzed Propylene Polymerization
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the entire course of metallocene (rac-Et(Ind)2ZrCl2)-catalyzed polymerization, forming solvent-complexed cation with thus-afflicted catalytic properties. Enlightened by these highly illuminating works, we have come up with a tentative scheme giving explanations to why and how copolymerization with (E,E,Z)-CDT will have a dramatic effect on stereo-selectivity of propylene polymerization with the typical “site-controlled” rac-Et(Ind)2ZrCl2/MAO catalyst. We believe, as illustrated in Figure 4, that due to its large steric bulkiness, (E,E,Z)-CDT the comonomer may be of very low reactivity for polymerization, which has matter-of-fact been confirmed by the copolymerization results. Under such a circumstance, the copolymerization system must encompass considerable amount of monomeric (E,E,Z)-CDT throughout the course of polymerization, which increases along with its feeding concentration increasing. With multiple nonconjugate vinyl moieties, (E,E,Z)-CDT is supposedly more active in electron donating than the conjugate toluene even though the latter is in higher quantity. Thus it is of great likelihood that, although unable to be succeeded by inserting into the metal–polymer bond, the unreacted (unenchained) (E,E,Z)-CDT will assume constant coordination to the Zr species (in equilibrium with that of propylene monomer) forming comonomer-complexed active site that is the de facto catalyst species charting the entire course of polymerization. Such a catalyst species is then reasonably expected of, alongside decreased activity due to electronic hindrance of (E,E,Z)-CDT, enhanced enantiomorphic selectivity for propylene polymerization due to steric hindrance that would be inevitably associated with the bulky comonomer. That, as we speculate, might constitute the major interpretation of the copolymerization results of propylene and (E,E,Z)-CDT especially on the drastic increments of PP isotacticity upon copolymerization. Nonetheless, considering the indeed incorporation of (E,E,Z)-CDT into PP chains despite the tiny amount, which is however unlikely to be at chain end, a minor contribution to the enhanced stereo-selectivity
as well as decreased catalyst activity might be due to a spatial suppression in the vicinity of the metal center exerted by the enchained (E,E,Z)-CDT that will also cause enhanced stereo-selectivity for subsequent propylene insertion. Since (E,E,Z)-CDT is such a bulky and rigid molecule, this incorporated comonomer effect may trickle down to several propylene insertions to come and eventually want to restore a full control of propylene insertion stereo-chemistry to the active site. However, we deem this as a minor factor only in compensation to the major (E,E,Z)-CDT coordinating effect, after all the comonomer incorporations have been too low to induce any significant effects.
3. Conclusions
In summary, with a cyclic triene (E,E,Z)-CDT, we show the first example that comonomer can enhance stereo-selectivity of C2-symmetric metallocene catalyst in propylene polymerization. With the most mundane rac-Et(Ind)2ZrCl2/ MAO catalyst system, copolymerization of (E,E,Z)-CDT with propylene at 40 °C results generally in a low comonomer incorporation rate. It is observed that (E,E,Z)-CDT incorporation proceeds through both its cis- and trans-vinyl moieties, however only one vinyl moiety is reacted for each incorporated (E,E,Z)-CDT unit. Despite the tiny (E,E,Z)-CDT incorporations, the copolymerizations are observed of significant decreases (however without halting) in polymerization efficiency and equally significant increases in PP isotacticity as revealed by drastic increments in mmmm fraction and no alteration on the overall PP stereosequences. In light of the peculiar structural character of (E,E,Z)-CDT, a comonomer-complexing hypothesis is proposed to account for the phenomenal comonomer effect, in which (E,E,Z)-CDT is suppose to coordinate to the Zr species (in equilibrium with that of propylene monomer) forming comonomer-complexed active site in charge of the entire polymerization reaction with decreased activity however increased propylene insertion enantiomorphic selectivity. Meanwhile, (E,E,Z)-CDT enchainment causing spatial suppression on the metal center might also contribute to the stereoselectivity enhancement during propylene polymerization, which however is deemed as a much minor factor due to the extremely low comonomer incorporations. However, we are also fully aware that such a sterically exerted stereoselectivity enhancement may not be Figure 4. Plausible mechanism of (E,E,Z)-CDT increasing catalyst’s stereo-selectivity for arbitrary and many factors, such as catalyst structure and polymerization propylene polymerization.
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conditions, will be very important in determining the role of (E,E,Z)-CDT in affecting the structure and properties of PP. Therefore, our subsequent research is focusing on altering polymerization temperature as well as catalyst structure to have an in depth, more comprehensive understanding of the unique, both expected and unexpected (E,E,Z)-CDT effects in its copolymerization with propylene. Those results will be reported elsewhere.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: Financial support by the National Science Foundation of China (Grant Nos. 20874104, 21374121, 51373178, 51003105, and 51103163) is gratefully acknowledged.
4. Experimental Section Materials: rac-Et(Ind)2ZrCl2 was purchased from Aldrich and used as received. MAO (abbreviation of methylauminoxane, 1.4 M in toluene) was purchased from Albemarle and used as received. (E,E,Z)-CDT was obtained from TCI and purified by drying over CaH2 followed by distillation. Polymerization-grade propylene was supplied by Yanshan Petrochemical Co. of China. AR grade toluene was obtained from Sinopharm Chemical Reagent Beijing Co. and distilled over sodium and benzophenone before use. Copolymerization: In a typical reaction (run 2 in Table 1), in a 250 mL three-necked flask equipped with a magnetic stirrer were performed the copolymerization reactions. Before starting the polymerization, the flask was conditioned by three vacuum/propylene purging/refilling cycles. Toluene (45.5 mL) and (E,E,Z)-CDT (0.025 mol, 4.5 mL) in a total volume of 50 mL were then syringed into the reactor. Propylene was fed into the flask at 40 °C under a constant pressure of 0.1 MPa which was maintained throughout the course of polymerization. Then 3 mL of MAO solution (1.4 M, 4 mmol) and 1 mL of rac-Et(Ind)2ZrCl2/toluene solution (2 × 10−3 M, 2 × 10−6 mol) were introduced into the reaction. The polymerization was allowed to proceed for 0.5 h. It was then terminated by the addition of a small amount of acidic ethanol. The polymer was precipitated upon pouring the whole reaction mixture into a large excess of ethanol. The precipitate was at last collected by filtration, washed again with ethanol, and finally dried in vacuum at 50 °C for 10 h, to give 2.5 g powdery product. Characterization: Room-temperature 1H NMR spectra were recorded on a Bruker AVANCE 400 spectrometer. All high-temperature 1H NMR and 13C NMR spectra were recorded on a Bruker DMX 300 spectrometer at 110 °C using o-dichlorobenzene-d4 as the solvent. Melting temperatures (Tm) and crystallization temperatures (Tc) of polymers were measured by DSC on a Pyris 1 Perkin-Elmer instrument operating at a scan rate of 10 °C min−1 under a flowing nitrogen atmosphere. The molecular weight and molecular weight distribution of the polymers were determined by gel permeation chromatography (GPC) using a Waters Alliance PL-GPC 220 instrument equipped jointly with a two-angle laser light scattering detector, a viscosity detector, and a differential refractive index detector. The measurement was performed at
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150 °C with 1,2,4-trichlorobenzene as the eluent at a flow rate of 1.0 mL min−1.
Received: December 25, 2014; Published online: February 26, 2015; DOI: 10.1002/marc.201400736 Keywords: copolymerization; C2-symmetric catalysts; propylene; stereo-selectivity; 1,5,9-cyclododecatriene
metallocene trans,trans,cis-
[1] C. E. Carraher Jr., Polymer Chemistry, 9th ed., CRC Press, Boca Raton, FL 2014. [2] N. Pasquini, Polypropylene Handbook, 2nd ed., Hanser, Munich 2005. [3] G. W. Coates, Chem. Rev. 2000, 100, 123. [4] F. R. W. P. Wild, L. Zsolnai, G. Huttner, H. H. Brintzinger, J. Organomet. Chem. 1982, 232, 233. [5] J. A. Ewen, L. Haspeslach, J. L. Atwood, H. Zhang, J. Am. Chem. Soc. 1987, 109, 6544. [6] W. Spaleck, M. Antberg, J. Rohrmann, A. Winter, B. Bachmann, P. Kiprof, J. Behm, A. W. Herrmann, Angew. Chem. Int. Ed. 1992, 31, 1347. [7] W. Spaleck, F. Kuber, A. Winter, J. Rohrmann, B. Bachmann, M. Antberg, V. Dolle, E. F. Paulus, Organometallics 1994, 13, 954. [8] W. Spaleck, M. Antberg, M. Aulbach, B. Bachmann, V. Dolle, S. Haftka, F. Kuber, J. Rohrmann, A. Winter, Ziegler Catalysts (Eds: G. Fink, R. Mulhaupt, H. H. Brintainger), SpringerVerlag, Berlin 1995, pp. 83–97. [9] M. C. Sacchi, F. Forlini, S. Losio, I. Tritto, U. M. Wahner, I. Tincul, D. J. Joubert, E. R. Sadiku, Macromol. Chem. Phys. 2003, 204, 1643. [10] U. M. Wahner, I. Tincul, D. J. Joubert, E. R. Sadiku, F. Forlini, S. Losio, I. Tritto, M. C. Sacchi, Macromol. Chem. Phys. 2003, 204, 1738. [11] L. Boggioni, A. Ravasio, C. Zampa, D. R. Ferro, I. Tritto, Macromolecules 2010, 43, 4532. [12] R. Radeglia, H. Poleschner, G. Haufe, Magn. Reson. Chem. 1993, 31, 1054. [13] L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 2000, 100, 1253. [14] F. Forlini, I. Tritto, P. Locatelli, M. C. Sacchi, F. Piemontesi, Macromol. Chem. Phys. 2000, 201, 401. [15] F. Forlini, E. Princi, I. Tritto, M. C. Sacchi, F. Piemontesi, Macromol. Chem. Phys. 2002, 203, 645.
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Communication
Comonomer-Induced Stereo-Selectivity Enhancement in a C2-Symmetric MetalloceneCatalyzed Propylene Polymerization Lin Ma, Jin-Yong Dong*
Propylene polymerization is carried out with a C2-symmetric metallocene catalyst of racEt(Ind)2ZrCl2/MAO at 40 °C in the presence of a cyclo-triene of trans,trans,cis-1,5,9-cyclododecatriene ((E,E,Z)-CDT). Comonomer incorporations are rather low (7% in [mmmm]). (E,E,Z)-CDT is speculated to coordinate to the metal center forming comonomer-complexed active sites in charge of the entire polymerization reaction with decreased activity however increased propylene enantiomorphic selectivity.
1. Introduction Stereochemistry control is one of the most basic issues in polymer chemistry.[1] In coordination polymerization of α-olefins, it is particularly important as, on one hand, coordination polymerization allows the synthesis of stereoregular poly(α-olefin)s (e.g., isotactic polypropylene) by regulating the enantiofaces of the monomers during their insertions, on the other hand, poly(α-olefin)s, or polyolefins, with high stereo-regularity are prone to crystallize
L. Ma, Prof. J.-Y. Dong CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China E-mail: [email protected] L. Ma University of Chinese Academy of Sciences, Beijing 100049, China Macromol. Rapid Commun. 2015, 36, 733−738 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
so as to make such polyolefins as polypropylene of weak interchain interaction really useful polymeric materials with remarkable mechanical- and thermal-resistant properties.[2] In general, stereo-selectivity of α-olefin coordination polymerization originates from the transition metal catalyst that enables the polymerization. For isospecific propylene polymerization enabling isotactic polypropylene, it is known that the development of C2-symmetric ansa-metallocene catalysts with an enantiomorphic-site control mechanism has been so successful that some of the state-of-the-art ones are now able to rival their commercial heterogeneous counterparts in producing highly isotactic polypropylenes ([mmmm] > 99%) only with far higher activities.[3–8] However, it has been reported, even with the sitecontrolled C2-symmetric ansa-metallocene catalysts, the stereo-selectivity of propylene insertions will be affected by the last-inserted monomer unit when a comonomer is introduced into the polymerization.[9–11] Sacchi et al.,
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for one instance, found that the presence of 1-pentene during propylene polymerization by rac-Me2Si(2-MeBenz[e]Ind)2ZrCl2/MAO would decrease its stereo-selectivity, forming copolymers of gradually reduced isotacticities.[9] It was proposed that the insertion of a higher α-olefin comonomer such as 1-pentene would trigger a shift of the catalyst active site from highly stereo-selective to less or even non-stereo-selective.[10] For another instance, Tritto and co-workers observed that copolymerization with norbornene also greatly reduced the tacticity of the PP blocks when propylene copolymerization was carried out with rac-Me2Si(2-Me-Ind)2ZrCl2/MAO.[11] Meanwhile, it was found that, with rac-Et(Ind)2ZrCl2 as catalyst, both copolymerizations have little effect on its stereo-selectivity, the formed polypropylene possessing comparable isotacticities as well as similar stereo-sequences when related to polypropylene homopolymer. Concerning the effect of copolymerization on the stereo-selectivity of C2-symmetric metallocene catalysts, it is however noticed that there has been so far no report of catalyst stereo-selectivity increasing by copolymerization. In this report, we design a special copolymerization of metallocene-catalyzed propylene polymerization using a bulky cyclo-triene of trans,trans,cis-1,5,9-cyclododecatriene ((E,E,Z)-CDT) as comonomer. With racEt(Ind)2ZrCl2/MAO as a catalyst complex, we will show the first example of increasing polypropylene tacticity by a comonomer of (E,E,Z)-CDT.
2. Results and Discussion The cyclo-triene comonomer, (E,E,Z)-CDT, is a multifunctional cyclic olefin taking a peculiar configuration as shown in Figure 1.[12] Its 1H NMR spectrum is included in Figure 2 (Figure 2a), where the characteristic chemical shifts can be grouped to high-field and low-field ones, corresponding, respectively, to the cyclic methylene and vinylic methine protons. The one cis double bond exhibits as a single proton resonance at 5.28 ppm, whereas the trans double bonds are revealed by two distinct peaks at 5.19 and 5.03 ppm. The copolymerization of propylene and (E,E,Z)-CDT was carried out in toluene with the catalyst system composed of rac-Et(Ind)2ZrCl2 and MAO. A relatively low temperature of 40 °C was chosen for the copolymerization for restraining the chain transfer reactions.[13] The concentrations of (E,E,Z)-CDT were gradually increased in the series of polymerization runs. The resultant polymer samples were worked up by a thorough washing with ethanol after they had been retrieved from the polymerization reactor, to assure complete removal of unreacted (E,E,Z)-CDT. Table 1 summarizes the polymerization conditions and results. The obtained copolymers were first examined by 1H NMR, the spectra of which are shown in Figure 2, where
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Figure 1. Structure of trans,trans,cis-1,5,9-cyclododecatriene, (E,E,Z)-CDT.
they are being compared with the monomeric (E,E,Z)-CDT as well as with the PP homopolymer (Figure 2b) to detect any possible (E,E,Z)-CDT incorporations. As demonstrated in Figure 2, although weak, the characteristic resonances of the vinylic protons of (E,E,Z)-CDT around 5.0–5.3 ppm can still be distinguished from interference by those of the vinylidene protons of the chain terminals resulting from β-H elimination. In fact, this set of peaks displays a similar contour as those in the monomeric form. The cis double bond protons are shown with a resonance signal around 5.3 ppm, which indicates enchainment of the trans double bond. Signals around 5.03 and 5.19 ppm are from the trans double bond protons. The emergence of all these resonance signals suggests that both the cis and trans double bonds of (E,E,Z)-CDT are able to be engaged in copolymerization with propylene. And a rough integration of the individual peaks reveals that the cis and trans double bonds maintain a 1/2 ratio as that in its monomeric form, from where it can be assumed that the two types of double bonds may have similar reactivity. Considering the large molecular bulk of (E,E,Z)-CDT which is expected of considerable steric hindrance, it is thus temporally assumed that for each incorporated (E,E,Z)-CDT only one double bond would be enchained. Thus also by peak integration, the relative contents of (E,E,Z)-CDT in polymers can be estimated, the results included in Table 1. It has to be admitted that, with the current catalyst system and under the current polymerization conditions, the incorporations of (E,E,Z)-CDT are very limited, the maximum being only 0.10 mol%. These tiny amounts of incorporations are easy to be mistaken as that unreacted monomers failed to remove during polymer work-up process. To clarify this ambiguity, all samples were subjected to rigorous boiling ethanol extraction for extensive hours before tested again by 1H NMR. The results turned out to be almost identical as the previous ones, which thus verifies the authenticity of (E,E,Z)-CDT incorporations in polymers, though again extremely small. Despite these low incorporations of (E,E,Z)-CDT, as apparent from Table 1, substantial losses of
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Figure 2. 1H NMR spectra of a) monomeric (E,E,Z)-CDT, b) pristine PP (run 1 in Table 1), and c–e) its (E,E,Z)-CDT copolymers with different (E,E,Z)-CDT incorporations. c) Run 2 in Table 1; d) run 3 in Table 1; e) run 4 in Table 1.
catalyst activity were encountered during the copolymerization, and the higher the (E,E,Z)-CDT feed in the copolymerization, the lower the catalyst activity. Molecular weights (Mw) as well as molecular weight distributions (PDI) of the copolymers were determined by
GPC. The results are also included in Table 1. It has been known that, in case of propylene/α-olefin copolymerization with the same catalyst system, the copolymer molecular weights would decrease with increasing of copolymer’s α-olefin content due to more frequent chain transfer
Table 1. Conditionsa) and results of copolymerization of propylene and (E,E,Z)-CDT with rac-Et(Ind)2ZrCl2/MAO. Run
CDT [mol L−1]
Yield [g]
Cat. activityb)
CDT in polymer [mol%]
Mwc) [g mol−1]
PDIc)
Tmd) [°C]
ΔHfd) [J g−1]
Tcd) [°C]
Stereo-pentade) [%] [mmmm]
[mmmr]
[mmrr]
[mrrm]
1
0
4.9
4.9
0
27 700
2.8
131.7
54.7
101.0
82.1
8.0
6.4
3.2
2
0.5
2.5
2.5
≈0.04
27 500
3.1
134.6
66.4
104.6
88.7
5.9
4.3
1.1
3
1.0
1.3
1.3
≈0.06
21 500
2.8
134.8
68.7
105.8
86.0
7.0
5.0
1.5
4
1.5
0.6
0.6
≈0.10
26 400
2.9
135.1
76.3
104.6
89.3
6.2
3.4
0.9
a) Zr = 2 μmol, molar ratio of (MAO) Al/Zr = 2000, CDT + toluene = 50 mL, propylene = 0.1 MPa, polymerization temperature = 40 °C, polymerization duration = 0.5 h; b)×106 g mol−1 Zr h; c)Determined by GPC; d)Determined by DSC; e)Determined by 13C NMR.
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reaction at α-olefin-inserted propagating chain end.[13] However, comparing the PP/(E,E,Z)-CDT copolymers with the PP homopolymer in Table 1, no notable decrease of Mw can be found. In fact, this is a highly valued information for it rules out such speculations as that the comonomer might be placed at the polymer chain end. Besides molecular weight, the copolymers also maintain relatively narrow molecular weight distributions. The GPC curves (shown in the Supporting Information) are smooth without any shoulder peaks. The Mark–Houwink plots (also shown in the Supporting Information) indicate that all copolymer samples exhibit straight line log[η] versus logMw relationship parallel with that of the homopolymer sample, suggesting that there is no branching structure formed in the copolymers, which thus confirms the previous hypothesis that each enchained (E,E,Z)-CDT has only one double bond, be it the cis or one of the two trans ones, engaged in the copolymerization. The PP block stereo-sequences in PP/(E,E,Z)-CDT copolymers were examined and quantified by 13C NMR, in comparison with those of the reference PP homopolymer (Figure 3). The copolymers display, similar to the PP homopolymer, three major stereo-irregular sequences including mmmr, mmrr, and mrrm, along with the stereoregular mmmm pentad. Interestingly, quantification of these stereo-sequences results in an unexpected finding that copolymerization with (E,E,Z)-CDT even increases isotacticity in PP, which in turn suggests stereo-selectivity enhancement for this otherwise well-known metallocene catalyst. Comparing the mmmm pentad percentage fraction in the whole stereo-sequences between the reference PP homopolymer and the PP/(E,E,Z)-CDT copolymers and
among the copolymers themselves, an up to 7.2% increment from 82.1% in pristine PP homopolymer to 89.3% in copolymer containing only 0.1 mol% of (E,E,Z)-CDT is evidenced enough to showcase the remarkable effect of (E,E,Z)-CDT the cyclo-triene as comonomer on the stereoselectivity of the metallocene-catalyzed polymerization. Increases in PP isotacticity lead to improved melt crystallization properties. Differential scanning calorimetry (DSC) thermogram comparison (shown in the Supporting Information) is made between the pristine PP homopolymer and PP/(E,E,Z)-CDT copolymers at both heating and cooling cycles. Some important data from this comparison are collected and included in Table 1. Again, with so small comonomer incorporations, melting temperatures (Tm) of PP/(E,E,Z)-CDT copolymers increase from 131.0 °C of PP homopolymer to 135.8 °C of copolymer with 0.1 mol% of (E,E,Z)-CDT enchainment. In accordance, melting fusion enthalpies (ΔHf ) increase from 54.0 to 76.8 J g−1, and crystallization temperatures (Tc) from 101.1 to ≈105.1 °C. These notable increments in crystallization properties are expected to induce significant improvements in mechanical and processing properties of ultimate PP applications. To summarize the copolymerization of propylene with (E,E,Z)-CDT under the as-adopted conditions, we have found that, despite the copolymerization has not been so successful since comonomer incorporations are all too limited, it is still of great scientific interest for it provides the very first example of comonomer inducing phenomenal stereo-selectivity enhancement in coordination α-olefin polymerization using “site-controlled” C2-symmetric metallocene catalyst. To account for this exceptional experimental finding, we quote two related previous works both by Sacchi et al. that are highly insightful in terms of explaining exotic influences in metallocene-catalyzed olefin polymerization.[9–11,14,15] In one of the two works they proposed,[9–11] as mentioned earlier, that in propylene/α-olefin (e.g., 1-pentene) copolymerization resolved by racMe 2 Si(2-MeBenz-[ e ]Ind) 2 ZrCl 2 /MAO, insertion of the α-olefin comonomer at the propagating polymer chain end would have a temporal influence on the active catalyst structure that affects the stereo-selectivity of the immediately followed several propylene monomer insertions (a downwards effect). In the other,[14,15] with abundant evidences, they have asserted polymerization medium, that is, solvent which is coordinative with donor character such as Figure 3. 13C NMR spectra at the methyl region of a) pristine PP (run 1 in Table 1) and b–d) its (E,E,Z)-CDT copolymers with different (E,E,Z)-CDT incorporations. b) Run 2 in toluene or mesitylene will pose competition with α-olefin monomers during Table 1; c) run 3 in Table 1; d) run 4 in Table 1.
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the entire course of metallocene (rac-Et(Ind)2ZrCl2)-catalyzed polymerization, forming solvent-complexed cation with thus-afflicted catalytic properties. Enlightened by these highly illuminating works, we have come up with a tentative scheme giving explanations to why and how copolymerization with (E,E,Z)-CDT will have a dramatic effect on stereo-selectivity of propylene polymerization with the typical “site-controlled” rac-Et(Ind)2ZrCl2/MAO catalyst. We believe, as illustrated in Figure 4, that due to its large steric bulkiness, (E,E,Z)-CDT the comonomer may be of very low reactivity for polymerization, which has matter-of-fact been confirmed by the copolymerization results. Under such a circumstance, the copolymerization system must encompass considerable amount of monomeric (E,E,Z)-CDT throughout the course of polymerization, which increases along with its feeding concentration increasing. With multiple nonconjugate vinyl moieties, (E,E,Z)-CDT is supposedly more active in electron donating than the conjugate toluene even though the latter is in higher quantity. Thus it is of great likelihood that, although unable to be succeeded by inserting into the metal–polymer bond, the unreacted (unenchained) (E,E,Z)-CDT will assume constant coordination to the Zr species (in equilibrium with that of propylene monomer) forming comonomer-complexed active site that is the de facto catalyst species charting the entire course of polymerization. Such a catalyst species is then reasonably expected of, alongside decreased activity due to electronic hindrance of (E,E,Z)-CDT, enhanced enantiomorphic selectivity for propylene polymerization due to steric hindrance that would be inevitably associated with the bulky comonomer. That, as we speculate, might constitute the major interpretation of the copolymerization results of propylene and (E,E,Z)-CDT especially on the drastic increments of PP isotacticity upon copolymerization. Nonetheless, considering the indeed incorporation of (E,E,Z)-CDT into PP chains despite the tiny amount, which is however unlikely to be at chain end, a minor contribution to the enhanced stereo-selectivity
as well as decreased catalyst activity might be due to a spatial suppression in the vicinity of the metal center exerted by the enchained (E,E,Z)-CDT that will also cause enhanced stereo-selectivity for subsequent propylene insertion. Since (E,E,Z)-CDT is such a bulky and rigid molecule, this incorporated comonomer effect may trickle down to several propylene insertions to come and eventually want to restore a full control of propylene insertion stereo-chemistry to the active site. However, we deem this as a minor factor only in compensation to the major (E,E,Z)-CDT coordinating effect, after all the comonomer incorporations have been too low to induce any significant effects.
3. Conclusions
In summary, with a cyclic triene (E,E,Z)-CDT, we show the first example that comonomer can enhance stereo-selectivity of C2-symmetric metallocene catalyst in propylene polymerization. With the most mundane rac-Et(Ind)2ZrCl2/ MAO catalyst system, copolymerization of (E,E,Z)-CDT with propylene at 40 °C results generally in a low comonomer incorporation rate. It is observed that (E,E,Z)-CDT incorporation proceeds through both its cis- and trans-vinyl moieties, however only one vinyl moiety is reacted for each incorporated (E,E,Z)-CDT unit. Despite the tiny (E,E,Z)-CDT incorporations, the copolymerizations are observed of significant decreases (however without halting) in polymerization efficiency and equally significant increases in PP isotacticity as revealed by drastic increments in mmmm fraction and no alteration on the overall PP stereosequences. In light of the peculiar structural character of (E,E,Z)-CDT, a comonomer-complexing hypothesis is proposed to account for the phenomenal comonomer effect, in which (E,E,Z)-CDT is suppose to coordinate to the Zr species (in equilibrium with that of propylene monomer) forming comonomer-complexed active site in charge of the entire polymerization reaction with decreased activity however increased propylene insertion enantiomorphic selectivity. Meanwhile, (E,E,Z)-CDT enchainment causing spatial suppression on the metal center might also contribute to the stereoselectivity enhancement during propylene polymerization, which however is deemed as a much minor factor due to the extremely low comonomer incorporations. However, we are also fully aware that such a sterically exerted stereoselectivity enhancement may not be Figure 4. Plausible mechanism of (E,E,Z)-CDT increasing catalyst’s stereo-selectivity for arbitrary and many factors, such as catalyst structure and polymerization propylene polymerization.
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conditions, will be very important in determining the role of (E,E,Z)-CDT in affecting the structure and properties of PP. Therefore, our subsequent research is focusing on altering polymerization temperature as well as catalyst structure to have an in depth, more comprehensive understanding of the unique, both expected and unexpected (E,E,Z)-CDT effects in its copolymerization with propylene. Those results will be reported elsewhere.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: Financial support by the National Science Foundation of China (Grant Nos. 20874104, 21374121, 51373178, 51003105, and 51103163) is gratefully acknowledged.
4. Experimental Section Materials: rac-Et(Ind)2ZrCl2 was purchased from Aldrich and used as received. MAO (abbreviation of methylauminoxane, 1.4 M in toluene) was purchased from Albemarle and used as received. (E,E,Z)-CDT was obtained from TCI and purified by drying over CaH2 followed by distillation. Polymerization-grade propylene was supplied by Yanshan Petrochemical Co. of China. AR grade toluene was obtained from Sinopharm Chemical Reagent Beijing Co. and distilled over sodium and benzophenone before use. Copolymerization: In a typical reaction (run 2 in Table 1), in a 250 mL three-necked flask equipped with a magnetic stirrer were performed the copolymerization reactions. Before starting the polymerization, the flask was conditioned by three vacuum/propylene purging/refilling cycles. Toluene (45.5 mL) and (E,E,Z)-CDT (0.025 mol, 4.5 mL) in a total volume of 50 mL were then syringed into the reactor. Propylene was fed into the flask at 40 °C under a constant pressure of 0.1 MPa which was maintained throughout the course of polymerization. Then 3 mL of MAO solution (1.4 M, 4 mmol) and 1 mL of rac-Et(Ind)2ZrCl2/toluene solution (2 × 10−3 M, 2 × 10−6 mol) were introduced into the reaction. The polymerization was allowed to proceed for 0.5 h. It was then terminated by the addition of a small amount of acidic ethanol. The polymer was precipitated upon pouring the whole reaction mixture into a large excess of ethanol. The precipitate was at last collected by filtration, washed again with ethanol, and finally dried in vacuum at 50 °C for 10 h, to give 2.5 g powdery product. Characterization: Room-temperature 1H NMR spectra were recorded on a Bruker AVANCE 400 spectrometer. All high-temperature 1H NMR and 13C NMR spectra were recorded on a Bruker DMX 300 spectrometer at 110 °C using o-dichlorobenzene-d4 as the solvent. Melting temperatures (Tm) and crystallization temperatures (Tc) of polymers were measured by DSC on a Pyris 1 Perkin-Elmer instrument operating at a scan rate of 10 °C min−1 under a flowing nitrogen atmosphere. The molecular weight and molecular weight distribution of the polymers were determined by gel permeation chromatography (GPC) using a Waters Alliance PL-GPC 220 instrument equipped jointly with a two-angle laser light scattering detector, a viscosity detector, and a differential refractive index detector. The measurement was performed at
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150 °C with 1,2,4-trichlorobenzene as the eluent at a flow rate of 1.0 mL min−1.
Received: December 25, 2014; Published online: February 26, 2015; DOI: 10.1002/marc.201400736 Keywords: copolymerization; C2-symmetric catalysts; propylene; stereo-selectivity; 1,5,9-cyclododecatriene
metallocene trans,trans,cis-
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